Apparatus including a bucking transformer for operating electric discharge lamps



Jui 1%, 3967 w. F. POWELL, m 3,331,987 APPARATUS INCLUDING A BUCKINGTRANSFORMER FOR OPERATING ELECTRIC DISCHARGE LAMPS Original Filed July25, 1962 INVENTOR,

MT" WALTER F POWELLQJ'FFH United States Patent 3 331,987 APPARATUSINCLUlllING A BUCKING TRANS- FORMER FOR OPERATING ELECTRIC DIS- CHARGELAMPS Walter F. Powell, Jr., Danville, Ill., assignor to GeneralElectric Company, a corporation of New York Original application July23, 1962, Ser. No. 211,554, now Patent No. 3,249,799, dated May 3, 1966.Divided and this application June 16, 1965, Ser. No. 464,517

3 Claims. (Cl. 315206) This invention relates generally to apparatus foroperating electric discharge devices, such as fluorescent lamps, withalternating current. More particularly, it relates to an improvedballasting and operating arrangement for such apparatus. Thisapplication is a divisional application of my copending applicationfiled July 23, 1962, Ser. No. 211,554 which issued May 3, 1966 as PatentNo. 3,249,799.

The voltage required to initiate current flow in an electric dischargelamp varies with the length and type of electric discharge lampoperated. Usually the voltage required to operate the electric dischargelamp when normal lamp current is flowing through the lamp is less thanthe starting voltage. If the lamp current increases during operation,the voltage drop across the lamp will decrease as lamp currentincreases. This tendency of the lamp voltage to vary inversely with thelamp current is generally referred to as its negative resistancecharacteristic.

It is, therefore, a requirement of an apparatus for operating electricaldischarge lamps that it provide some means for limiting the currentsupplied to the lamp. If the current supplied to the lamp is not limitedby some means, the current will continue to build up until the lamp isdestroyed. A well-known way of limiting the current supplied to anelectric discharge device, such as a fluorescent lamp, is to provide aballasting resistor in series with the lamp.

The fluorescent lamp may be operated in a series loop arrangement whichincludes the ballasting resistor, the power source, and the lamp. Inorder to provide for stable operation and appropriate regulation of thelamp, the voltage drop across the ballasting resistor generally is aboutequal to the normal operating voltage of the fluorescent lamp. If thedifference between the starting voltage and the normal operating voltageof the lamp in such a resistive ballasting arrangement is small, slightchanges in the supply voltage would produce appreciable variations inthe light output of the lamp. It is, therefore, necessary inapplications where a resistor is used as a ballasting element to providea voltaged drop across the ballasting resistor that is about equal tothe normal operating voltage of the lamp.

Where the fluorescent lamp is operated in a series loop arrangement witha ballasting resistor, it will be appreciated that the vector sum of thevoltage drop across the ballasting resistor and the voltage drop acrossthe lamp is equal to the supply voltage. Since the supply voltage isgenerally maintained at a substantially constant level, as the lampcurrent builds up because of the inherent negative resistancecharacteristic of the lamp, the current through the ballasting resistorincreases, This results in a proportional increase in the voltage dropacross the ballasting resistor thereby causing the voltage across thelamp to decrease. Conversely, when the lamp current decreases, thevoltage across the ballasting resistor decreases thereby causing thelamp voltage to increase. In this manner, the current supplied to thelamp is eflectively limited.

Resistive elements have not been generally used in alternating currentballasting systems since they dissipate an appreciable amount of power.Reactive type of ballasting devices have been widely used since theyconsume less power than a ballasting resistor. Since reactive devices donot impede the flow of direct current, reactive ballasting elements havenot been used in direct current systems for ballasting. However,resistors have been used in direct current systems despite therelatively large power losses occurring in the resistor.

A principal disadvantage of conventional resistive ballasting systems isthat the power consumed by the ballasting resistor is generally aboutthe same as that required to operate the lamp. Thus, the efliciency ofthe system is about fifty percent. It is desirable, therefore, to reducethe power losses in a resistive type of ballast While achievingsatisfactory regulation and stability. Further, it is desirable toprovide an apparatus for operating electric discharge lamps that doesnot require a large difference between the lamp starting voltage (opencircuit voltage) and the lamp operating voltage. It will be appreciatedthat as the difference between the starting voltage and the operatingvoltage is reduced, less energy is required to be dissipated or storedin the ballasting elements. Consequently, the components in the systemcan be smaller in size and weight, and where a ballasting resistor isemployed, less power is dissipated in the resistor.

Accordingly, it is a general object of the present invention to providean improved apparatus for operating electric discharge devices.

A more specific object of the present invention is to provide animproved apparatus for operating electric discharge lamps, such asfluorescent lamps, wherein the lamp can be operated with a relativelysmaller dilference between the lamp starting voltage and the lampoperating voltage.

It is another object of the present invention to provide an improvedapparatus for operating a fluorescent lamp that utilizes a resistivetype of ballasting and can be operated at relatively greater efliciencythan conventional ballasting systems employing resistors as ballastingelements.

In accordance with one form of my invention, I have provided an improvedapparatus for operating at least one electric discharge lamp, such as afluorescent lamp, from an alternating power source that employs avariable impedance network arrangement. The network arrangement providesan instantaneously varying impedance during a portion of each half cycleto control the current supplied to the electric discharge lamp in orderto prevent the lamp from destroying itself because of its negativeresistance characteristic. In the preferred form of my invention, thevariable impedance network includes at least one transistor that isdriven to provide an instantaneously variable impedance to control thelamp current, and a relatively low impedance is provided during an earlyand late portion of each half cycle. The emitter and collectorelectrodes are connected in circuit with the output terminals of afull-wave bridge rectifier. Base drive for thetransistor may be obtainedfrom the full-wave bridge rectifier or may be obtained from a separatesource, such as a feedback source, a variable D.C. supply or a fixedD.C. supply. The input terminals of the bridge rectifier may be placeddirectly in the lamp circuit, or if it is desired to employ transistorshaving relatively lower voltage ratings, a transformer may be interposedbetween the bridge rectifier and the lamp circuit.

In another form of my invention, I have provided a variable impedancebridge network for controlling the current supplied to a fluorescentlamp which is comprised of a full-wave rectifier, a pair of transistors,and a transformer. One of the windings of the transformer is connectedin circuit with at least one output lead and input lead of the apparatusto place the variable impedance bridgenetwork in series circuit vwiththe lamp during operation. The other of the transformer windings isconnested in circuit with the collector electrodes of the transistorsand has a tap connected in circuit with the output terminals of thefull-.wave rectifier. Further, a resistor may be connected in circuitwith the other of the output terminals and in circuit with the emitterelectrodes of the transistors to function as a current measuringelement. A bias supply means is connected in circuit with the baseelectrodes of the transistors. The transistors are drivenby the biassupply means to provide an instantaneously variable impedance in theprimary circuit of'the transformer whereby the current supplied to thelamp is regulated.

According to another aspect of the, invention, the bias supply means iscomprised of a transformer having a primary and a center tappedsecondary Winding. The primary winding is connected to a suitable signalsource. For example, the transformer may be connected across the outputleads of the apparatus where it isdesired to sense the voltage or inseries with the lamp where it is desired to sense the lamp current or toa separate source having a predetermined wave shape where it is desiredto provide a lamp current with a corresponding wave shape. Further, thecentertap ofthe secondary winding is connected to one of the outputleads of the fullawave bridge rectifier, and the ends of the secondarywin-ding are con-v nected to the base electrodes f the pair oftransistors to supply base drive current thereto.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with otherobjects and advantages may be better understood by referring to thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIGURE 1 is a schematic circuit diagram of an apparatus embodying a formof my invention wherein the variable impedance bridge network providesthe bucking 'voltage to stabilize the lamp and is activated in responseto feedback signal from lamp, circuit; and

FIGURE 2 illustrates another embodiment of my inventionin which thevariable impedance bridge network of my invention also supplies thevoltage to operate a fluorescent lamp.

In FIGURE 1, I have illustrateda form of my invention in which avariable impedance bridge network is employed to dynamically vary theprimary voltage of a.

bucking transfromer T 7 connected in the lamp circuit. The impedanceintroduced in the circuit is high during a portion of each half cycle sothat the bucking transformer presents no appreciable impedance to lampcurrent flow. During the portion of each half cycle when the electricdischarge lamp conducts current, one of the pair of transistors Q, or Qprovides' a varying impedance in the primary circuit of the buckingtransformer T7 to vary its voltage and thus regulate the lamp: current,as will hereinafter be more fully described.

The apparatus 80 embodying this form of my invention is shown enclosedin a dashed rectangle 81 which sub stantially represents the enclosurefor the apparatus 80. A pair of input terminal leads 82 and 83ov isprovided for connection to a suitable alternating-current supply. Theoutput of the apparatus 80 is applied across the lamp 84 by means ofoutput leads 85 and 86.

A bucking voltage is introduced into the lamp circuit during operationbya variable impedance network. This network is comprised of atransformer T having a pair of primary windings P P and a secondarywinding 8 a full wave rectifier 87 including diodes D D D and Dtransistors Q7, Q and a current transformer T having a primary winding Pand secondary windings S S inductively coupled therewith. The secondarywinding S .of transformer T is closely coupled with the primary windingsP P so that transformer T has a low magnetizing reactance and does notimpede the curthat are applied across the emitter-base junctions oftran-v sis'tors Q and Q In each half cycle of the alternating currentsupply one ofthe transistors Q7, Q will be forward biased and the otherwill be reverse biased. When one of the transistors Q Q conducts, aportion of the output of the full wave rectifier 87 is applied acrossone of the primary windings P or P to induce a voltage across thesecondary winding S that is in an opposing or bucking relation with theinstantaneous voltage applied across input terminal leads 82, '83.

Operation of the apparatus is initiated by connecting the input terminalleads 82, 83 in circuit with a suitable power supply, such as a volt, 60cycle alternating current supply. During the open circuit condition, allof source voltage is applied across output leads 85, 86, and lamp 84will ionize and conduct current. When lamp 84 conducts current, currentwill also flow through the primary winding P of the current transformerT Let us take an arbitrary alternation of the alternating current supplywhen the polarity of the voltage is such that the input terminal lead 83is positive with respect to input terminal lead 82. The path of currentflowwill be from input terminal lead 83, tooutput lead 8,5,the lamp 84,the primary P the secondary S and input terminal lead 82. The currentflow through the primary winding P causes a voltage to be applied acrossthe winding P and the polarity of this voltage will be such that thelower end, of the winding P as seen in FIGURE 1, is positive withrespect to the upper end. Accordingly, a voltage. is.

87 is applied across primary winding P depending uponthe magnitude ofthe current in primary winding P The base current drive to transistors QQ, can be balanced if necessary by sliding thetap which divides thesecondary into windings S and S or by connecting a resistor in serieswith each base electrodeof transistors Q7, Q8

Current will now flow through lead 89, the diode'D lead 91,: transistorQ primary winding P lead 90, diode D lead 88'and to input terminal lead82. As a result, a bucking voltage is induced across the secondarywinding S of transformer T The polarity of this bucking voltage is suchthat the lower end is positive with respect to the upper end of thewinding as will be seen in FIG- URE 1.

During the positive alternation of the power supply, the bucking voltageis controlled in the following manner: If the feedback signal or drivecurrent supplied by current transformer T to the on transistor Q isinsufficient to drive it to saturation, a voltage dropappears across theemitter and collector electrodes of the transistor Q As the currentthrough the primary'winding P1 of the current transformer T increases,the base drive current to transistor Q also increases. Hence, as thebase drive current increases, the impedance of transistor Q decreases,and also the voltagedrop across the transistor Q decreases.Consequently, the voltage across the primary winding 'P increases andcauses the bucking voltage across secondaryS to increase. Thus, an,increase in lamp current results in an increase in the bucking voltage,and the current supplied to the lamp 84 is reduced. Similarly, adecrease in the current flow through the primary winding P brings abouta decrease in the bucking voltage across the secondary winding S In thismanner, regula tion and control of the current supplied to the lamp 84is achieved by varying the impedance connected in circuit with theprimary winding P of the bucking transformer T During the negativealternation of the power supply, the secondary winding S the transistorQ and the primary winding P come'into play since the polarity of thevoltage induced across the secondary windings S S is reversed. Thepolarity of this voltage is such that the lower end of secondary windingS is negative, and consequently the transistor Q is now forward biased.

Under normal operating conditions of the apparatus 80 the base drivecurrent supplied to the transistor Q; is insufficient to drive it tosaturation. Thus, the output voltage of the bridge rectifier 87 isproportionally divided across transistor Q and the primary winding P Asthe current through the primary winding P increases, the base drivecurrent to transistor Q increases. Accordingly, the voltage drop acrossthe emitter and collector electrodes of transistor Q decreases therebycausing the voltage across the primary winding P of transformer T toincrease. As during the positive half cycle, the voltage appearingacross the secondary S increases in response to an increase in lampcurrent and decreases with a decrease in lamp current therebycontrolling the current supplied'to the lamp 84.

Referring now to FIGURE 2, I have shown therein another form of myinvention embodying a variable impedance network for controlling andsupplying the current required for operation of an electric dischargelamp 99. The apparatus for operating the electric discharge lamp 99 isgenerally identified by reference numeral 100 and is shown enclosed in adashed rectangle 101. The apparatus 100 is energized by connecting apair of input leads 102 and 103 across a suitable alternating currentsource. The output of the apparatus 100 is supplied to the electricdischarge lamp 99 by output leads 104 and 105.

As will hereinafter be more fully explained, the var iable impedancebridge network arrangement in the exemplification of the invention shownin FIGURE 2 will reproduce across output leads 104, 105 a currentcorresponding in waveshape to the waveshape of a feedback signal appliedacross a pair of feedback leads 106, 107 or, in other words, across theprimary winding P of transformer T The transformer T has a pair ofsecondary windings S and S inductively coupled with the primary windingP on a magnetic core 108.

It will be noted that the input leads 102 and 103 are connected with theinput terminals of a full wave bridge rectifier 109 which includesdiodes D 7, D D and D One of the output terminals of the bridgerectifier 109 is connected by lead 110 to the tap to which primarywindings P and P of transformer T are joined. The other output terminalof bridge rectifier 109 is connected in circuit with the emitterelectrodes of transistors Q and Q through a resistor R and leads 111,112 and 113 and is also connected with secondary windings S S by lead114.

Continuing with the description of apparatus 100 shown in FIGURE 2, theoperation will now be more fully described. In order to start theoperation of the apparatus 100, the input terminal leads 102 and 103were connected to an A.C. power source and the feedback leads 106 and107 were also connected with an A.C. power supply through a smallfilament transformer T to apply a sinusoidal signal across the feedbackleads 106 and 107.

Let us arbitrarily assume that the voltage across the primary winding Pat a given instant is such that the upper end of the Winding P as seenin FIGURE 2, is negative with respect to the lower end. As a result, the

voltage induced across the secondary windings S and S is such that theupper end is negative with respect to the lower end. A negative voltageis now applied at the base electrode of the transistor Q, to switchtransistor Q into conduction. At this instant, substantially the entireoutput voltage of the bridge rectifier 109 is applied across primary Pand a voltage is induced across the secondary winding S of thetransformer T Assuming that this instantaneous voltage is suflicient toionize lamp 99, lamp 99 will begin to conduct. Consequently, currentbegins to flow in the loop which includes lamp 99, output lead 104,secondary winding S and output lead 105.

A current flow through the secondary winding S in effect, lowers theresistance reflected to the primary winding P Consequently, more currentis supplied by the power source through the bridge rectifier 109.However, the increased current flow produces a voltage drop acrossresistor R This current is allowed to build up until the voltage dropacross the resistor R approximately equals the potential applied at thebase of transistor Q at which time the base drive on the transistor Q;will be insufficient to support additional current flow. When thisoccurs, the voltage across the resistor R will in effect track thevoltage applied at the base electrode of transistor Q Also, transistor Qhas a voltage drop that is substantially equal to the difference betweenthe supply voltage and voltage developed across the transformer T Iflamp 99 tries to draw more current than the value corresponding to thelimited voltage developed across the resistor R the transistor impedanceincreases and the voltage drop across the collector and emitterelectrodes of transistor Q; will increase thereby causing the voltageapplied to the primary winding P to decrease. Similarly, when the lampcurrent decreases, this voltage drop will decrease and cause the voltageacross the secondary winding S to increase. In this manner, the currentto lamp 99 is dynamically controlled by the varying impedance introducedby the transistor Q If the secondary winding S is short circuited, thecurrent in the circuit is still effectively limited by the voltage dropacross the resistor R and by the voltage available at the base electrodeof transistor Q In this case, the voltage developed across transformer Tis zero. On the other hand, when output leads 104, are open circuited,the full rectified output of the power source is made available acrossthe primary winding P to provide the maximum voltage across thesecondary winding S When the voltage across the primary winding Preverses, it will be understood that the lower end of the secondarywinding S is now negative with respect to the upper end, as seen. inFIGURE 2, and a negative voltage now appears at the base electrode oftransistor Q During this alternation of the power source, primarywinding P provides the driving voltage for transformer T and the loopwhich includes transistor Q resistor R and the primary winding P comeinto play. In the same manner as during the previous alternation of thepower supply, a decreasing current flow through the secondary winding Shas the effect of lowering the resistance reflected into the primarywinding P and thereby causing more current to be supplied thereto fromthe power source through bridge rectifier 109. As this current flow inthe loop increases, the voltage drop across the resistor R increases.The current is allowed to build up until the voltage drop across theresistor R is nearly equal to the potential at the base electrode oftransistor Q At this point, the base drive on transistor Q will beinsufficient to support additional current flow through the transistor QIf the lamp circuit now attempts to draw more current, the voltage dropacross the collector and emitter electrode of transistor Q willincrease. Thus, the voltage across the primary winding P will decrease,and lamp operating voltage across the secondary winding S decreases. Inthis way, the lamp current is limited by the varying impedance of thetran- SiStOl' Q10.

Although, in the above described exemplification of the invention, thefeedback signal was an alternating signal having a substantiallysinusoidal waveshape, and in phase with the power source, it will beappreciated that other signals of different waveshapes may be providedto drive the variable impedance bridge circuit of the apparatus 100. Forexample, if the voltage across the transformer secondary winding S isfed back to the feedback leads 106 and 107, the signal will have thewaveshape of the lamp voltage, and the lamp current waveshape will becontrolled to correspond with the lamp voltage waveshape. With such anarrangement, it will be apparent that unity lamp power factor can beachieved. Further, it will be appreciated that with the variableimpedance bridge network arrangement shown in FIG- URE 2, any desiredwaveshape of the lamp current can be obtained since the apparatus 100will essentially provide a current in the secondary winding having awaveshape corresponding to the waveshape of the voltagesignalappliedacross theprirnary winding P From the foregoing description ofthe various exemplifications of the invention, it will be apparent thatthe ballasting action for one or more fluorescent lamps is provided by avariable impedance network. This network introduces an instantaneouslyvariable impedance, which may regulate lamp current indirectly as in theembodiments shown in FIGURES 1 and 2. An important advantage of theinvention is that the variable impedance network makes it possible tominimize losses inthe circuit that would otherwise result if a linearresistor were used as the ballasting element. 'Also, the variableimpedance network arrangement makes it possible. to design an apparatusfor operating electric discharge lamps with a smaller difference betweenthe supply voltage and the operating voltage of the lamp than would bethe case if conventional ballasting elements were employed in thecircuit to provide the current limiting action for the electricdischarge lamps. Further, the variable impedance network of theinvention is readily adaptable to control by a signal responsive to thelamp operating condition. A signal sensing an operating condition or asignal from an independent source may beemployed to control thewaveshape of the lamp current.

Although a variable impedance network utilizing a bridge has beenemployed in the exemplification of my invention, it will be'apparent tothose skilled in the art that variable impedance networks employingbilateral semiconductor devices or unidirectional devices in an inversearrangement may be used in the practice of the invention. Itwill beunderstood that the specific exemplifications of the invention which Ihave described herein are intended for illustrative purposes only andthat many modifications may be made. It is, therefore, intended by theappendedclaims to cover all such modifications that fall within the truespirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A ballast apparatus comprising a pair of input leads for connectionin circuit with an alternating power source, output leads for connectionin circuit with at least one electric discharge lamp, a buckingtransformer having a center tapped primary winding and a secondarywinding, a current transformer having a primary winding and a centertapped secondary winding, said secondary Winding of said buckingtransformer and said primary winding of said current transformer beingconnected in circuit with one .of said output leads and one of saidinput terminal leads for connection in series circuit relation with saidlamp, a full wave bridge rectifier having input terminals and outputterminals, said input terminals being connected in circuit with saidinput terminalleads for connection with the alternating power source, apair of transistors, each of said transistors having a collector, an

emitter, and a base electrode,-one of said output termie nals of saidbridge rectifier being connected in circuit with the tap of the primarywinding of said bucking transformer, the other of said terminals beingconnected in circuit with the emitter electrodes of said transistors andthe tap of said secondary winding of the current transformer, circuitmeans connecting the ends of the primary winding of said buckingtransformer in circuit with the collector electrodes of said transistorsand connecting the ends of the secondary winding. of vsaid currenttransformer in circuit with the base electrodes of said transistors,said transistors providing a variable impedance in the primary circuitof said bucking transformer to regulate the current'flow through thesecondary, winding of the bucking transformer in:response tochanges inthe current supplied atthe output leads.

2. Ballast apparatus comprising a pair of input leads for connection toan alternating power source, a pair of output leads for connection to atleast one electric discharge lamp, a bucking transformer having aprimary winding and-a secondary winding coupled thereto, currentindicating means having input terminals and output terminals, meanscoupling said secondary winding of said bucking transformer and saidinput terminals of said current indicating means in circuit with one ofsaid input leads and one of said output leads for connection in seriescircuit relations with said lamp, a rectifier having input terminals andoutput terminals, said rectifier input terminals being connected incircuit with said input leads for connection with said alternating powersource, at least one transistor having an input electrode and outputelectrodes, means coupling one of said rectifier output terminals tosaid primary winding of said bucking transformer, means coupling theother of said rectifier output terminals to one of said outputelectrodes of said tran-,

sistor, means coupling said primary winding of said bucking transformerto at least the other output electrode of said transistor, andmeanscoupling said outputterminals of said currentindicating means to saidinput electrode of said transistor to regulate the current flow throughsaid secondary winding of said bucking transformer in response tochanges in'current through said input terminals of said currentindicating means as indicated by said output terminals of said currentindicating means.

3. Ballast apparatus comprising a pair of input leads for connection toan alternating current power source, a pair'of output leads forconnection to at least one electric discharge lamp, a buckingtransformer having a tapped primary winding and a secondary windingcoupled thereto, current indicating means having input terminals andoutput terminals, means coupling said secondary winding of said buckingtransformer and said input terminals of said current indicating means incircuit with one of said input leads and one of said output leads forconnection in series circuit relation with said lamp, means coupling theother oflsaid input leads'to the other of said output leads, a full waverectifier having input terminals and output terminals, said inputterminals being connected in circuit with said input leads forconnection with said alternating power source,'a pair-of transistorseach having an input electrode and output electrodes, means coupling oneof said output terminals of said rectifier to said tap on said primarywinding of said bucking transformer, means coupling the other of saidoutput terminals of said rectifier to corresponding output electrodes of9 10 cated by said output terminals of said current indicating 2,923,8562/1960 Greene et al. 315l38 means. 3,193,726 7/1965 Powell 315199References Cited 3,249,799 5/ 1966 Powell 31598 UNITED STATES PATENTSJAMES W LAWRENCE P E 2,748,274 5/1956 Pearlman 331-114 5 xammer'2,843,815 7/ 1958 Driver 331-114 X C. R. CAMPBELL, Assistant Examiner.

1. A BALLAST APPARATUS COMPRISING A PAIR OF INPUT LEADS FOR CONNECTION IN CIRCUIT WITH AN ALTERNATING POWER SOURCE, OUTPUT LEADS FOR CONNECTION IN CIRCUIT WITH AT LEAST ONE ELECTRIC DISCHARGE LAMP, A BUCKING TRANSFORMER HAVING A CENTER TAPPED PRIMARY WINDING AND A SECONDARY WINDING, A CURRENT TRANSFORMER HAVING A PRIMARY WINDING AND A CENTER TAPPED SECONDARY WINDING, SAID SECONDARY WINDING OF SAID BUCKING TRANSFORMER AND SAID PRIMARY WINDING OF SAID CURRENT TRANSFOMER BEING CONNECTED IN CIRCUIT WITH ONE OF SAID OUTPUT LEADS AND ONE OF SAID INPUT TERMINAL LEADS FOR CONNECTION IN SERIES CIRCUIT RELATION WITH SAID LAMP, A FULL WAVE BRIDGE RECTIFIER HAVING INPUT TERMINALS AND OUTPUT TERMINALS, SAID INPUT TERMINALS BEING CONNECTED IN CIRCUIT WITH SAID INPUT TERMINAL LEADS FOR CONNECTION WITH THE ALTERNATING POWER SOURCE, A PAIR OF TRANSISTORS, EACH OF SAID TRANSISTORS HAVING A COLLECTOR, AN EMITTER, AND A BASE ELECTRODE, ONE OF SAID OUTPUT TERMINALS OF SAID BRIDGE RECTIFIER BEING CONNECTED IN CIRCUIT WITH THE TAP OF THE PRIMARY WINDING OF SAID BUCKING TRANSFORMER, THE OTHER OF SAID TERMINALS BEING CONNECTED IN CIRCUIT WITH THE EMITTER ELECTRODES OF SAID TRANSISTORS AND THE TAP OF SAID SECONDARY WINDING OF THE CURRENT TRANSFORMER, CIRCUIT MEANS CONNECTING THE ENDS OF THE PRIMARY WINDING OF SAID BUCKING TRANSFORMER IN CIRCUIT WITH THE COLLECTOR ELECTRODES OF SAID TRANSISTORS AND CONNECTING THE ENDS OF THE SECONDARY WINDING OF SAID CURRENT TRANSFOMER IN CIRCUIT WITH THE BASE ELECTRODES OF SAID TRANSISTORS, SAID TRANSISTORS PROVIDING A VARIABLE IMPEDANCE IN THE PRIMARY CIRCUIT OF SAID BUCKING TRANSFORMER TO REGULATE THE CURRENT FLOW THROUGH THE SECONDARY WINDING OF THE BUCKING TRANSFORMER IN RESPONSE TO CHANGES IN THE CURRENT SUPPLIED AT THE OUTPUT LEADS. 